> How, in practical terms, do you run an economy with completely open
> borders?
You can start by not sweating the small stuff. The borders of the US are essentially open at this point anyway; it's just that with the constant threat of hassle by the government, those who are here "illegally" are effectively barred from participating fully (cue to the Governator again veto'ing the bill to allow drivers licences). The fact is: the economy is running just fine _despite_ these quasi-open borders. The underground labor economy is probably a net lose in the US.
Some people say they can't imagine "just showing up" and getting US citizenship; I say it's hard to understand how just being born here gives you that position, no questions asked. It seems unlikely that native-born-to-citizens newborns will be given some kind of test to pass any time soon, so why should anyone else?
The best expression of the fears behind open borders comes from people like Pat Buchannan. Isn't that enough evidence to convict?
/jordan